1. Field of the Invention
The present invention relates to a method of determining an exposure time for a photographic printer and, more particularly, to such a method which may be applied to a photographic printer capable of varying an enlargement or print magnification (the ratio of a length of a print to the corresponding length of the negative image) to determine optimum exposure time which corresponds to a print magnification set during printing.
2. Description of the Prior Art
One type of photographic printer has conventionally been known in which frame images recorded on an original film are enlarged or magnified, usually enlarged, and projected onto sheets of print paper to produce magnified prints. With such a photographic printer, the print magnification is varied to produce a magnified print of a predetermined size. However, when the print magnification is varied, the dimension of the image projected onto the sheet of print paper changes. Accordingly, if the printing is effected while the exposure time is kept unchanged, the optimal print would not be obtained. To overcome this problem, it has been the conventional practice to calculate the exposure time from the following formula: T=(1+m).sup.2 /4 (where T represents the exposure time; and m represents the print magnification), to thereby determine the optimum exposure time corresponding to the print magnification set in the photographic printer. This method of determining an exposure time is based on the fact that the quantity of light per unit area is in inverse proportion to the square of the magnification.
If, however, the printer is a photographic printer for manual printing which is often applied to commercial use, prints must be printed with an increased level of precision. In this case, therefore, if an exposure time is determined on the basis of a fixed calculation formula, such as that mentioned above, prints would not be printed properly regardless of the emulsion number coated on the print paper, though this may depend on the characteristics of the print paper used.
In order to cope with this problem, another method of determining an exposure time has been used in particular with a photographic printer for manual printing. This method is adapted to conduct a plurality of test printings using different print magnifications and measure an exposure time to ensure the optimum print with each print magnification. The relationship between the exposure time and the print magnification is approximated to a linear relationship from the print magnifications used in the test printings and the exposure times measured. Then, a calculation is made on the basis of this linear relationship and, an exposure time corresponding to a print magnification is set for actual printing, so as to effect the printing.
With this conventional method of determining the exposure time, however, the relationship between the exposure time and the print magnification is determined as a linear relationship. Consequently, although proper prints can be provided with a print magnification within a certain region of magnification, proper prints cannot be provided with a print magnification outside this region. Thus, the conventional method fails to ensure proper prints with respect to the entire range of magnification extending from a region for reduction to a region for enlargement, thereby frequently forcing the photographer to restart his printing operation.